Electrical and Electrochemical Properties of Carbon Coating for PEMFC Metallic Bipolar Plate

Abstract

i.e., significant buckling of the C/Ti-bilayer-coated STS 316L did not occur during the scratch test. The Ti interlayer balanced the thermal expansion coefficients (CTEs) between the substrate and the carbon film, since the Ti has intermediate CTE value. And owing to its higher bond energy with carbon, compared with that of the C and STS 316L, the Ti can also anchored the carbon overcoat. The ICR value of the C/Ti bilayer coating also satisfied the DOE target. This value depended primarily on the quality of interfacial contact between the GDL and the bipolar plate. However, the bulk conductivity of Ti is extremely low and has negligible influence on the ICR. Although the columnar-structured Ti interlayer led to an open structure of the carbon overcoat, the corrosion resistance of the C/Ti bilayer coating still fulfilled the DOE target. In fact, the high-potential stability increased substantially. When potentials larger than 0.8 VSCE were applied to the sample during the potentiostatic polarization test, the STS 316L bipolar plate underwent pitting corrosion by Cr ionization. This pitting deteriorated the adhesion of the carbon single-layer coating and cracks propagated along the interface between the carbon and the substrate. However, the passive oxide state of Ti persisted at these high potentials, thereby resulting in a high-potential stability of the C/Ti bilayer coating. In brief, the carbon coating on STS 316L exhibited excellent properties and are therefore well-suited for use in bipolar plates. Moreover, the Ti interlayer improved the adhesion of the carbon coating as well as the high-potential stability.

impedance spectra measured before and after the potentiostatic polarization at 0.6 VSCE were compared. The spectrum of the carbon film deposited at ?60 V bias changed only slightly after the corrosion test, and reflected the low anodic current density. In the case of the carbon film deposited at 0 V bias, the capacitive behavior at low frequencies became resistive owing to pore enlargement. The substrate bias therefore reduced the coating porosity and improved the corrosion resistance. The adhesion of the carbon film was inadequate for the protection of the STS 316L bipolar plate. The carbon film grown at a high substrate bias was able to store large amounts of elastic energy. This stored energy was released via film buckling and delamination during the scratch test. To overcome the adhesion problem, a titanium interlayer was inserted between the STS 316L substrate and the carbon film. The mechanical adhesion was improved by the Ti interlayer

thick and fragile graphite plates are ill-suited for such applications. Reducing the volume and weight of the stack as well as the stack cost is therefore essential to the commercialization of PEMFCs. These reductions can be achieved by replacing the graphite plate with metallic ones. As such, in this project, amorphous carbon film was deposited on an STS 316L bipolar plate using inductively coupled plasma (ICP) assisted magnetron sputtering. The coated samples all satisfied the DOE requirement that the interfacial contact resistance (ICR) values must be lower than 20 mΩ?cm2. After potentiostatic polarization, the surface of the carbon films and the ICR values changed only slightly. In addition, the surface roughness of the coating increased with increasingly negative applied bias voltage, which resulted in decreased ICR values. The corrosion current densities of the carbon film deposited at zero and negative bias voltages were larger and lower, respectively, than the DOE target of 1 μA?cm-2. In fact, the films exhibited a lowest corrosion current density of 0.28 μA?cm-2 when the carbon was coated at a bias of ?60 V. The carbon coating produced at a 0 V bias had a higher oxygen content, and led to a higher cathodic current density

this current density resulted from the high catalytic effect of the oxygen content in the film. The EIS analysis with an equivalent circuit model revealed the porosity corrosion mechanism of the anodic current regime

Polymer electrolyte membrane fuel cells (PEMFCs) operate at low temperatures and have quick start-up abilities as well as high power densities. PEMFCs are the most promising candidate for use in portable electronics, transportation vehicles, and small-scale stationary plants. Owing to its high electrical conductivity and chemical stability, machined graphite is typically used for the bipolar plate of PEMFCs. However, the graphite plate is brittle and therefore cannot be sufficiently thinned in order to realize a high power density and low weight. PEMFCs designated for use in vehicles require more than 500 bipolar plates